Anaerobic bacteria reductively dechlorinate polychlorinated biphenyls (PCBs) in aquatic sediments, but these microorganisms remain uncultured and, until now, unidentified. Through denaturing gradient gel electrophoresis (DGGE) of 16S rDNA from a highly enriched ortho-PCB dechlorinating culture, the growth of a single microorganism was shown to be dependent upon the presence and dechlorination of 2,3,5,6-tetrachlorobiphenyl. This is the first identification of a microorganism that catalyses the reductive dechlorination of a PCB. The organism, bacterium o-17, has high sequence similarity with the green non-sulphur bacteria and with a group that includes Dehalococcoides ethenogenes. Bacterium o-17 required acetate for dechlorination and growth. H2:CO2 (80:20 at 101 kPa) did not support dechlorination or growth of the dechlorinator. Archaeal 16S rDNA was not detected in actively dechlorinating bromoethanesulphonate-treated non-methanogenic cultures, which indicated that methanogenic Archaea were not required for dechlorination. The consistent association with dechlorinating activity combined with high similarity to other known dechlorinating microorganisms indicates that bacterium o-17 catalyses the reductive ortho-dechlorination of 2,3,5,6-tetrachlorobiphenyl.